FIG. 1A is a simplified block diagram schematically representing a typical prior art network router 10. Packet forwarding router 10 includes two major subsystems: control plane 12 and data plane 11. Data plane 11 provides the packet forwarding function in interfaces 14 for store-and-forward transit packets. This subsystem relies on a packet forwarding “look-up” table that is created and maintained by control plane 12. The forwarding table instructs data plane 11 where to forward each received packet. Control plane 12 creates the packet forwarding table using information from various sources, including static configuration and dynamic information learned from peer network routers through connections 15-1 through 15-N and interfaces 14, and communicates the forwarding tables to interfaces 14. In prior art systems, command line interface (CLI) 13 is a text-based system associated with control plane 12 for applying configuration changes to a router's operational state.
Open shortest path first (OSPF) is a dynamic routing protocol that is part of the router's control plane (see for example Doyle, “Routing TCP/IP,” Vol. I, MacMillan 1998, pp. 409-584, hereby incorporated herein by reference). It communicates with other OSPF instances running on other routers in a network to learn about remote destinations it can reach. OSPF contributes this information to the packet forwarding table used by the data plane.
OSPF is a link-state dynamic routing protocol, essentially containing three layers, as illustrated in FIG. 1B. The first layer is Hello protocol layer 180. OSPF periodically transmits Hello packets to each attached network. The function of a Hello packet is to discover new OSPF neighbor routers and to maintain relationships (called “adjacencies”) with existing OSPF neighbor routers. The second layer is Reliable Flooding layer 185. Each OSPF router originates messages called link-state advertisements (LSAs) that describe its network interfaces and adjacencies with other OSPF routers. The collection of all LSAs in the network is called the link-state database (LSDB). The OSPF Reliable Flooding protocol operates over the neighbor adjacencies to ensure that the link-state database is synchronized on all routers (i.e., that each router has all LSAs originated by all other routers). The third layer is Shortest Path First (SPF) layer 190 based on the Dijkstra SPF algorithm (see Doyle 1998, cited above). The SPF algorithm uses the LSDB as input to calculate packet forwarding table information. In general, OSPF executes the SPF algorithm whenever a change in the LSBD occurs.
FIG. 1C schematically illustrates the logical structure of a conventional OSPF global network 100, each logical entity of which has its own set of attributes. An OSPF area, for example areas 102-0, . . . , 102-N, is a logical grouping of OSPF routers 10-1, . . . , 10-N. Each area 102-0, . . . , 102-N is described by its own link state database, for example LSDB 104. Area border routers (ABRs) 111-1, 111-2 are routers that each belong to at least two areas 102-1, . . . , 102-N, connecting to area 102-0 (backbone area), and maintaining a separate LSDB, including running the SPF algorithm, for each such connected area. Neighbor routers 10-2, . . . , 10-N are linked to respective areas 102-0, . . . , 102-N through interfaces 103-1, . . . , 103-N. The OSPF routers on each multi-access (M-A) network segment 115, for example an Ethernet or ATM configuration, elect a Designated Router (DR) 110-1, which maintains links with all routers internal to its M-A segment. Each such M-A segment also selects a Backup Designated Router (BDR) 110-2, which becomes the new DR in case of failure of the existing DR. The other routers within a M-A segment maintain direct links only with DR 110-1 and BDR 110-2 internal to the M-A segment to which they belong and not with any other router in that M-A segment. DR 110-1 also manages the Reliable Flooding process on behalf of all routers in its M-A segment. This reduces the proliferation of copies of the same LSA on the same network that would otherwise occur and cause network traffic congestion. There can be more than one M-A segment 115 in a particular OSPF area. Virtual link 113 is an information “tunnel” not restricted to any particular physical link, through which OSPF protocol packets can be routed on the optimal path from one router to another. In case a link failure, for example at location 114, creates a partitioned area with isolated OSPF routers, a virtual link reconnects the OSPF routers in the partitioned area to the network backbone.
In the prior art, typically if an OSPF router fails, a separate standby peer router having its separate control plane and data plane is booted and then takes over from scratch, reconfigures itself, and re-establishes all of its interfaces with the network. The resulting network topology change affects other OSPF routers in the network, and leads to LSDB changes, new SPF calculations to create new forwarding tables, and possibly temporarily unreachable network destinations. It would therefore be advantageous in the art to develop a system and method for seamless fail-over of an OSPF, such that a standby entity takes over from a failed active entity without needing to replace, reinitialize, or reconfigure the failed router or its network connections.